Various types of omni-wheels are known. An omni-wheel can travel in both longitudinal and lateral directions by using a composite wheel which typically comprises a hub member configured to be rotatably supported and a plurality of free rollers each rotatably supported by the hub member around a rotational center line extending tangentially to a circle concentric to the rotational center line of the hub member and passing through a center of the free roller. See JP3381848B, JP2010-6352A and JP2010-195245A, for instance.
The free rollers are thus arranged on a concentric circle one next to the other such that the gap between each adjacent pair of free rollers is greater in a radially outer part than in a radially inner part. Therefore, the gap in the radially outer part cannot be reduced beyond a certain limit without causing some difficulty in supporting the free rollers because the gap in the radially inner part becomes extremely small. The large gap in the radially outer part prevents a smooth rotation of the hub member around the rotational center line thereof (or a smooth lateral travel of the omni-wheel), and causes undesired vibrations and noises.
This problem could be alleviated by increasing the diameter of the hub member and/or reducing the diameter of each free roller. Increasing the diameter of the hub member undesirably increases the size of the omni-wheel and the necessary number of free rollers so that a compact design cannot be achieved, and the manufacturing cost increases. If the diameter of each free roller is reduced, a smooth longitudinal travel is prevented. This impairs the performance of the omni-wheel.